In a color image forming apparatus, a color image is formed by superimposing a plurality of images in different colors, and therefore so-called color misregistration occurs if formation positions of images in different colors are misaligned with respect to desired positions. As such color misregistration degrades the image quality, a color misregistration correction mechanism is necessary. U.S. Pat. No. 8,837,994 suggests detection of a color misregistration amount through formation of a pattern, and calculation of a correction amount for correcting color misregistration. Such color misregistration occurs due to, for example, expansion and shrinkage of components of an image forming apparatus.
While various types of paper are used in an image forming apparatus, a fixing heat amount differs depending on paper types. For example, a heat amount necessary for thick paper is larger than a heat amount necessary for standard paper. Hence, the image forming apparatus has a mode in which an image is formed at an image forming speed lower than an image forming speed applied to standard paper. It is known that a color misregistration amount attributed to expansion and shrinkage of optical components does not depend on an image forming speed. Therefore, once the image forming apparatus has calculated a correction amount for correcting color misregistration through formation of a pattern at the image forming speed for the standard paper, the calculated correction amount can be used mutually at all image forming speeds.
In recent years, paper types are becoming diverse, and the number of image forming speeds that can be set in an image forming apparatus is increasing accordingly. That is to say, the range of image forming speeds used in an image forming apparatus is becoming wider. As the range of image forming speeds has widened, it has been discovered that color misregistration attributed to deterioration of components involved in conveyance of sheets of paper and images is evident. For example, a driving roller that drives an intermediate transfer belt undergoes abrasion, and the intermediate transfer belt deteriorates by getting dirty from scattered toner. This may cause the intermediate transfer belt to slip with respect to the driving roller, in which case timings of transfer from photosensitive drums of different colors to the intermediate transfer belt are shifted, and color misregistration occurs. It has been discovered that a change in a slip amount corresponding to the state of deterioration of the intermediate transfer belt depends on an image forming speed. That is to say, a slip amount at the lowest image forming speed is larger than a slip amount at the highest image forming speed. Therefore, if color misregistrations at all image forming speeds are corrected using a color misregistration correction amount that has been decided on based on the highest image forming speed, a color misregistration amount becomes large especially at the lowest image forming speed. Conversely, if color misregistrations at all image forming speeds are corrected using a color misregistration correction amount that has been decided on based on the lowest image forming speed, a color misregistration amount becomes large especially at the highest image forming speed.